In Nicholson Baker's new novella, "Checkpoint," a man sits in a Washington hotel room with a friend and talks about assassinating President Bush. It's a work of the imagination and no attempts on the president's life are actually made, but the novel is likely to be incendiary, as with Michael Moore's documentary, "Fahrenheit 9/11." Flush with the headline-generating success of "My Life," by Bill Clinton, Alfred A. Knopf is planning to publish Baker's work Aug. 24, on the eve of the Republican National Convention. "Checkpoint" is 115 pages long and will sell for $18.